This invention relates generally to methods and apparatus for magnetic resonance imaging (MRI) systems, and more particularly to methods and apparatus that facilitate maintaining a consistent shim temperature.
Typical superconducting magnets may use one or the combination of two independent systems to adjust the magnetic field homogeneity in the imaging volume, in order to meet image quality requirements. The first system is an active system that consists of superconducting correction coils embedded in the superconducting magnet cartridge. The second is a passive system that consists of the placement of carbon steel plates at specified axial and radial location from the magnet isocenter.
Both systems have design capability limitations that are driven by parameters such as: for active systems; maximum current, superconductor type, coils size, coils location and former structural integrity, and for passive systems; steel shim size, steel chemical composition homogeneity, steel shim location, and temperature. For gradient embedded magnet passive shimming system, steels shims are placed inside of the gradient coil in a tray and rail configuration that allows easy shim removal or shim replacement during magnet shimming operation. Heat provided by the gradient coil operation produces temperature fluctuation in the shims during imaging acquisition. For the embedded gradient shim configuration, heat travels from the coil structure to the shim due to conduction contact, by convection only if air is allowed at the shim location, and by radiation. For a magnet inner bore configuration, heat travels from the outside gradient surface to the shim by radiation and conduction thru supporting members if vacuum is present between the components.
Changes to the temperature of the shims causes a change to the magnet field center frequency due to changes in the magnetization properties of the steel with temperature (approximate 0.4%), this is called B0 intensity drift. The actual drift experienced by a magnet during an imaging scan, directly depends on the amount of steel used for the passive shimming, and the amount of heat that the shims are exposed to during gradient operation (shim temperature change). For good image quality, application specialists desire that the magnet drift cause less than a 1 pixel shift during a fifteen minute scan (a typical is 1 Hz/min). In order to achieve good image quality, one desire is that the temperature of the shim must not exceed a certain temperature rise per unit time. Therefore, an additional temperature control system or an active cooling system is typically employed to ensure that the magnet performance is maintained during any operation condition of the gradient coil. The additional cooling typically employed raises costs and adds complexity. Therefore, it would be desirable to maintain the shim's temperature without an expensive cooling system.